The present invention generally relates to adsorbents for carbon dioxide and, more specifically, to an improved adsorbent for carbon dioxide that can be particularly useful in environments having a highly concentrated but low partial pressure of carbon dioxide.
Future space exploration requires the ability to process resources available locally for the life support system of astronauts and for the propellant material that will send the astronaut and/or equipment back to Earth. On the surface beyond Earth, there may be highly concentrated but low partial pressure of carbon dioxide. The carbon dioxide can be a source of oxygen and propellant component for human habitat and the return of astronaut and equipment. For the application of this unique resource, the first step is the acquisition and concentration of carbon dioxide. One method to accomplish the first step is the adsorption of carbon dioxide during night time when there is low temperature and CO.sub.2 partial pressure. Next is the desorption/recovery of carbon dioxide during day time when there is higher temperature and to produce CO.sub.2 of high pressures. Such a diurnal cycle is described, for example, in Finn et al., "Utilisation of Martian Atmosphere Constituents by Temperature-Swing Adsorption," Journal of The British Inerplanetary Society, Vol 49, pp. 423-430, 1996.
A key to the success of the above adsorption/desorption technique is the working capacity of the adsorbent. Conventional adsorbents for the adsorption of carbon dioxide have been zeolite materials, such as 13X and 5A. These adsorbent materials are inorganic oxides that also adsorb water. Zeolites have been shown to have an adsorption capacity of 13% at 196.degree. K (which approximates the night time temperature of some environments). However, the binding energy of the carbon dioxide on the zeolite is high. To remove the carbon dioxide from the zeolite at a carbon dioxide partial pressure of 810 mm Hg (which approximates the pressure required for subsequent CO.sub.2 processing), a temperature of 523.degree. K with a residual carbon dioxide capacity of 2% is required. This results in a working capacity of only 11% at a desorption temperature of 523.degree. K. Such a low working capacity requires a large quantity of adsorbent for in-situ propellant production. Yet, weight, volume and energy are critical issues in the use of any equipment for space exploration.
In an effort to overcome disadvantages presented by a zeolite adsorbent, carbon-based materials have been used. For example, in U.S. Pat. No. 4,820,681 which is assigned to the assignee of the present invention, a carbon molecular sieve was prepared by polymerizing a cross-linking agent and a precursor monomer to produce a cross-linked polymer. The cross-linked polymer was then shaped into a desired configuration without the need for a binder. The shaped polymer was then carbonized.
In a fashion related to the above patent, U.S. Pat. No. 4,810,266 which is also assigned to the assignee of the present invention discloses a carbon molecular sieve. The sieve is similarly prepared by polymerizing a cross-linking agent and precursor monomer. The cross-linked polymer that is produced was also shaped into a desired configuration and carbonized. However, the pores of the material were then enlarged by steam treatment. And the material was given an amine functionality that improved capacity upon regeneration of the material.
While the above art has provided advantages, it has not adequately addressed the importance of adsorbent characteristics such as pore size, micropore volume and pore size distribution. These characteristics are important because carbon dioxide adsorption on solid adsorbents is based on the interaction forces between the gaseous molecule and the surfaces on micropores in the adsorbent.
As can be seen, there is a need for an improved adsorbent for carbon dioxide. There is also a need for an adsorbent for carbon dioxide that can be particularly useful in environments having a highly concentrated but low partial pressure of carbon dioxide. A further need is for a carbon dioxide adsorbent that has increased working capacity, especially at temperatures of around 196.degree. K and 423.degree. K which respectively represents an environmental night time temperature and a day time temperature. Similarly, a carbon dioxide adsorbent is needed that has increased working capacity at carbon dioxide partial pressures of around 6 mm Hg and 810 mm Hg--the approximate night time and process partial pressures in certain environments. Another need is for a carbon dioxide adsorbent for use in a diurnal cycle system wherein carbon dioxide is adsorbed and then desorbed.